Corrosion damage is a costly problem for environmentally exposed metals. Thus, people have attempted many solutions to prevent or reduce the effects of corrosion.
Chemically, metallic corrosion may be described as a coupled electrochemical reaction consisting of anodic metal oxidation and cathodic oxidant reduction. Metallic materials corrode in a variety of gaseous and/or aqueous environments, such as wet air in the atmosphere. In particular, metallic materials are susceptible to corrosion due to galvanic coupling, i.e., when two materials of different electrochemical potential (e.g., dissimilar metals) are electrically connected in the presence of an electrolyte (e.g., water with dissolved salt). Galvanic corrosion can occur on a macro scale, such as between different metallic components, or on a micro scale, such as between intermetallic phases at the surface of an alloy.
Corrosion protection may take a variety of forms, including utilizing corrosion-resistant metal alloys, isolating dissimilar metals, applying chemical conversion coatings, plating metals, and applying sealants (e.g., paint, epoxy, polyurethane). While in use, additional moisture barriers, such as viscous lubricants and/or protectants, may be added to a corrodible surface.
Conventional surface treatment for metals may use hexavalent chromium as the active corrosion-inhibiting ingredient. Though effective, environmentally preferred alternatives to hexavalent chromium are being sought. However, hexavalent chromium alternatives typically have several limitations including low corrosion suppression efficacy, poor compatibility with common coating materials, and high cost.
Sealants generally protect the underlying metal from corrosion by isolating the metal from the environment. If the integrity of the sealant coating is compromised, for example, because the coating does not adhere well to the metal or because the coating is damaged (cracked, scratched, etc.), the underlying metal may be exposed to corrosive conditions. Complicating the threat of corrosion due to loss of sealant integrity, sealants typically are opaque and mask the metal surface. Hence, corrosion that begins due to loss of sealant integrity may be hidden and may progress unnoticed.
The coupling of organic sealants (such as paint, epoxy, polyurethane, etc.) to metal may be enhanced by applying an adhesion promoter (also called a primer and/or a pretreatment) to the metal surface before applying the sealant. For example, silicon-zirconium-based sol-gels (e.g., 3M™ Surface Pre-Treatment AC-131) are commonly used to promote sealant adhesion without resorting to chromate conversion coatings, but with a lack of corrosion inhibition.
Thus, there exists a need for improved, and/or more environmentally preferred corrosion-inhibiting coating systems and methods that also may enhance sealant performance.